The sinuses are hollow spaces located inside the bones in the skull to either side of the nose, behind and in between the eyes, in the forehead and at the back of the nasal cavity. The sinuses are lined with a moist, thin layer of tissue called a mucous membrane, which not only humidifies the air as you breathe it in, but also produces mucus to trap irritants such as dust, pollen and bacteria. The sinuses are lined with microscopic hairs called cilia. The function of cilia is to move mucus to flush the sinuses and nasal passageways of trapped irritants.

Sinus congestion is the blockage of one or more of the nasal passageways as a result of inflammation and swelling of the sinus tissues, secretion of mucus or a deviated septum (meaning obstruction of the nasal passage by the membranous ridge of cartilage in the nose that separates the nasal cavity into the two nostrils). Sinus congestion leads to impaired flow of mucus out of the sinuses. The build up of mucus in the sinuses causes increased pressure. Also, bacteria can become trapped and infect the mucous membrane, a condition termed sinusitis.